Ultrasonic sensor having transmission device and reception device of ultrasonic wave

ABSTRACT

An ultrasonic sensor for detecting an object includes: a substrate; a transmission device for transmitting an ultrasonic wave; a plurality of reception devices for receiving the ultrasonic wave; and a circuit for processing received ultrasonic waves, which are received by the reception devices after the ultrasonic wave transmitted from the transmission device is reflected by the object. The transmission device and the reception devices are integrated into the substrate. The dimensions of the sensor are minimized, and detection accuracy of the sensor is improved.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2005-56639filed on Mar. 1, 2005, the disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to an ultrasonic sensor.

BACKGROUND OF THE INVENTION

An ultrasonic sensor is mounted on an automotive vehicle, for example.The sensor detects a distance between the sensor, i.e., the vehicle andan obstruction when a driver parks the vehicle or when the driver turnsthe vehicle. The ultrasonic sensor is disclosed in, for example,JP-A-2001-16694. The sensor for detecting the obstruction includes atransmission device and a reception device, which transmits anultrasonic wave and receives the ultrasonic wave. The sensor may includea transmitting/receiving device. When the transmission device transmitsthe ultrasonic wave, the ultrasonic wave hits the obstruction. Theobstruction reflects the ultrasonic wave; and then, the reflectedultrasonic wave is received by the reception device. On the basis of thereceived ultrasonic wave by the reception device, an acoustic pressureof the ultrasonic wave, a time lag and/or a phase difference aredetected so that a direction to the obstruction and a distance betweenthe obstruction and the vehicle are calculated. Further, a concavity anda convexity of the obstruction can be detected.

The reception device of the ultrasonic wave is for example, anultrasonic element having a vibrator formed of a piezoelectric thin filmdisposed on a membrane as a thin portion of a substrate. The ultrasonicelement with a membrane structure is disclosed in, for example,JP-A-2003-284182. This element is formed by a micro machining method sothat the element is called a MEMS (i.e., micro electro mechanicalsystem) type ultrasonic element. JP-A-2003-284182 also discloses anultrasonic array sensor including the MEMS type ultrasonic elements.

The MEMS type ultrasonic element 90R is shown in FIG. 13A. In theelement 90R, a PZT ceramics thin film layer 2 as a ferroelectricsubstance is sandwiched by a pair of electrodes 3 a, 3 b. The element90R further includes a piezoelectric sensor having a predeterminedresonance frequency for detecting the ultrasonic wave. When the element90R operates, a predetermined bias voltage is applied between twoelectrodes 3 a, 3 b so that the resonant frequency of the element 90R ischanged, i.e., controlled.

FIG. 13B explains a positioning measurement method by using theultrasonic wave, which is disclosed in JP-A-2003-284182. An ultrasonicsensor 900 includes an ultrasonic wave source 40 as a transmissiondevice of the ultrasonic wave and an ultrasonic array device A90R as areception device of the ultrasonic wave. The ultrasonic array deviceA90R includes multiple MEMS type ultrasonic elements 90R, which arearrayed. In the sensor 900, the source 40 is adjacent to the sensingdevice A90R, and transmits the ultrasonic wave. The ultrasonic wave hitsan object 51, 52 as an obstacle; and then, the ultrasonic wave isreflected by the object 51, 52. Thus, the ultrasonic wave is returned tothe sensor 900. The returned ultrasonic wave is received by each sensingelement 90R in the sensing device A90R. On the basis of the receivedultrasonic wave, the position of the object 51, 52 including anorientation angle to the object 51, 52 is determined. Specifically, onthe basis of a transmission time of the ultrasonic wave in each incidentdirection of the sensing element 90R, the distance between the sensingelement 90R and the object 51, 52 in the incident direction iscalculated. Thus, distribution of the distance in different incidentdirections is determined. Accordingly, the distance between the object51, 52 and the sensing element 90R in a depth direction of the object51, 52 is determined. Here, the transmission time of the ultrasonic waveis a time from a transmission time when the ultrasonic wave istransmitted from the source 40 to a returning time when the ultrasonicwave is returned to the sensing element 90R.

Here, the source 40 and the sensing device A90R are separated eachother. Therefore, a manufacturing cost of each of the source 40 and thesensing device A90R is necessitated. Further, when the source 40 and thesensing device A90R are mounted on a bumper of the vehicle, mountingaccuracy of each of the source 40 and the sensing device A90R affectsdetection accuracy of the direction and the distance of the object.Furthermore, the mounting distance between the source 40 and the sensingdevice A90R may be increased.

Further, in general, when an ultrasonic sensing device is directlymounted on the bumper of the vehicle, the sensing device cannot detectthe distance to the object accurately by a water drop or a dust attachedon a surface of the sensing element. Furthermore, attenuation of theultrasonic wave transmitting through air depends on temperature andhumidity of the air. These temperature and humidity are changeable inaccordance with the environment around the vehicle. Thus, the detectionaccuracy of the object may depend on temperature change and humiditychange. Specifically, the environmental temperature around the vehiclecan be detected by an external temperature sensor or the like. However,there is no appropriate external humidity sensor mounted on the outsideof the vehicle. Thus, the environmental humidity around the vehiclecannot be detected.

SUMMARY OF THE INVENTION

In view of the above-described problem, it is an object of the presentinvention to provide an ultrasonic sensor having a transmission deviceand a reception device of an ultrasonic wave.

An ultrasonic sensor for detecting an object includes: a substrate; atransmission device for transmitting an ultrasonic wave; a plurality ofreception devices for receiving the ultrasonic wave; and a circuit forprocessing received ultrasonic waves, which are received by thereception devices after the ultrasonic wave transmitted from thetransmission device is reflected by the object. The transmission deviceand the reception devices are integrated into the substrate.

The dimensions of the above sensor are minimized, compared with aconventional sensor. Further, a manufacturing cost of the sensor isreduced. Furthermore, a positioning relationship between thetransmission device and the reception device is accurately determined;and therefore, detection accuracy of the sensor is not affected bymounting accuracy of the sensor.

Alternatively, the number of the reception devices may be equal to orlarger than three so that the circuit is capable of detecting anoperation failure. Further, each of the transmission device and thethree reception devices has a surface for transmitting or receiving theultrasonic wave, the surface being perpendicular to a ground. The threereception devices are composed of a first to a third reception devices.The first reception device is disposed above the third reception device,and disposed on a left side of the second reception device. The circuitis capable of calculating a horizontal plane distance between the objectand the sensor in a horizontal plane parallel to the ground and ahorizontal plane direction angle from the sensor to the object in thehorizontal plane on the basis of the received ultrasonic waves receivedby the first and the second reception devices. The circuit is furthercapable of calculating a vertical plane distance between the object andthe sensor in a vertical plane perpendicular to the ground and avertical plane direction angle from the sensor to the object in thevertical plane on the basis of the received ultrasonic waves received bythe first and the third reception devices. The circuit is capable ofchecking the horizontal and the vertical plane distances and thehorizontal and the vertical plane direction angles on the basis of thereceived ultrasonic waves received by the second and the third receptiondevices so that the circuit is capable of detecting the operationfailure.

Alternatively, the number of the reception devices may be equal to orlarger than four. Further, each of the transmission device and the fourreception devices has a surface for transmitting or receiving theultrasonic wave, the surface being perpendicular to a ground. The fourreception devices are composed of a first to a fourth reception devices.The first reception device is disposed above the third reception device,and disposed on a left side of the second reception device. The fourthreception device is disposed under the second reception device, anddisposed on a right side of the third reception device. The circuit iscapable of calculating a horizontal plane distance between the objectand the sensor in a horizontal plane parallel to the ground and ahorizontal plane direction angle from the sensor to the object in thehorizontal plane on the basis of the received ultrasonic waves receivedby the first and the second reception devices, and further capable ofcalculating a vertical plane distance between the object and the sensorin a vertical plane perpendicular to the ground and a vertical planedirection angle from the sensor to the object in the vertical plane onthe basis of the received ultrasonic waves received by the first and thethird reception devices, so that a first data of the object is obtained.The circuit is capable of calculating the horizontal plane distance andthe horizontal plane direction angle on the basis of the receivedultrasonic waves received by the third and the fourth reception devices,and further capable of calculating the vertical plane distance and thevertical plane direction angle on the basis of the received ultrasonicwaves receive by the second and the fourth reception devices, so that asecond data of the object is obtained. The circuit is capable ofchecking the first data and the second data so that the circuit iscapable of detecting the operation failure.

Alternatively, the transmission device may be capable of transmittingmultiple ultrasonic waves having different frequencies so that thecircuit is capable of compensating humidity. Further, the transmissiondevice is capable of a first ultrasonic wave having a first frequencyand a second ultrasonic wave having a second frequency. The number ofthe reception devices is equal to or larger than three. Each of thetransmission device and the three reception devices has a surface fortransmitting or receiving the ultrasonic wave, the surface beingperpendicular to a ground. The three reception devices are composed of afirst to a third reception devices. The first reception device isdisposed above the third reception device, and disposed on a left sideof the second reception device. The circuit is capable of calculating ahorizontal plane distance between the object and the sensor in ahorizontal plane parallel to the ground and a horizontal plane directionangle from the sensor to the object in the horizontal plane on the basisof the received ultrasonic waves having the first frequency received bythe first and the second reception devices, and further capable ofcalculating a vertical plane distance between the object and the sensorin a vertical plane perpendicular to the ground and a vertical planedirection angle from the sensor to the object in the vertical plane onthe basis of the received ultrasonic waves having the first frequencyreceived by the first and the third reception devices. The circuit iscapable of calculating a first attenuation loss between the transmittedultrasonic wave and the received ultrasonic waves having the firstfrequency. The circuit is capable of calculating a second attenuationloss between the transmitted ultrasonic wave and the received ultrasonicwaves having the second frequency. The circuit is capable of calculatingthe humidity of environment on the basis of the first and the secondattenuation losses and a temperature obtained from an externaltemperature sensor.

Alternatively, each of the transmission device and the reception devicesmay be provided by an ultrasonic element. The ultrasonic element isdisposed on a membrane of the substrate. The ultrasonic element includesa piezoelectric thin film and a pair of metallic electrodes so that apiezoelectric vibrator is provided. The piezoelectric thin film issandwiched by the metallic electrodes. The piezoelectric vibrator iscapable of resonating together with the membrane at a predeterminedultrasonic frequency. Further, the piezoelectric thin film of thetransmission device includes a partial cutting pattern, which isdisposed on a stress concentration region of a radial directionvibration of the membrane. Furthermore, the membrane is separated by thepartial cutting pattern into four pieces. The membrane has a squareplanar shape, and each piece of the membrane has a square planar shape.The partial cutting pattern penetrates one of the metallic electrodesand the piezoelectric thin film.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1A is a top view showing an ultrasonic sensor according to apreferred embodiment of the present invention, and FIG. 1B is aschematic perspective view showing the sensor mounted on a circuitboard;

FIG. 2A is a plan view showing an ultrasonic element in the sensor, andFIG. 2B is a cross sectional view showing the element taken along lineIIB-IIB in FIG. 2A;

FIG. 3 is a plan view showing another ultrasonic sensor, according to amodification of the embodiment;

FIG. 4A is a plan view showing an ultrasonic element according to asecond modification of the embodiment, and FIG. 4B is a cross sectionalview showing the element taken along line IVB-IVB in FIG. 4A;

FIG. 5A is a plan view showing an ultrasonic element according to athird modification of the embodiment, and FIG. 5B is a cross sectionalview showing the element taken along line VB-VB in FIG. 5A;

FIG. 6A is a plan view showing an ultrasonic element according to afourth modification of the embodiment, and FIG. 6B is a cross sectionalview showing the element taken along line VIB-VIB in FIG. 6A, and FIG.6C is a partially enlarged cross sectional view showing a part VIC ofthe element in FIG. 6B;

FIG. 7A is a plan view showing an ultrasonic element according to afifth modification of the embodiment, and FIG. 7B is a cross sectionalview showing the element taken along line VIIB-VIIB in FIG. 7A;

FIG. 8A is a plan view showing an ultrasonic element according to asixth modification of the embodiment, FIG. 8B is a cross sectional viewshowing the element taken along line VIIIB-VIIIB in FIG. 8A, and FIG. 8Cis a cross sectional view showing the element taken along lineVIIIC-VIIIC in FIG. 8A;

FIG. 9A is a schematic view explaining a reception ultrasonic wave in aX-Y plane received by reception devices, FIG. 9B is a schematic viewexplaining the reception ultrasonic wave in a Z plane received by thereception devices, and FIG. 9C is a timing chart showing signals from atransmission device and four reception devices;

FIG. 10 is a timing chart showing signals having two differentfrequencies from a transmission device and four reception devices,according to a seventh modification of the embodiment;

FIG. 11A is a top view showing an ultrasonic sensor according to aneighth modification of the embodiment, and FIG. 11B is a timing chartshowing signals having two different frequencies from two transmissiondevices and four reception devices, according to the eighthmodification;

FIG. 12 is a top view showing an ultrasonic sensor according to a ninthmodification of the embodiment; and

FIG. 13A is a partially enlarged cross sectional view showing anultrasonic element according to a prior art, and FIG. 13B is a schematicview explaining a method for detecting an object by using an ultrasonicwave, according to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An ultrasonic sensor 100 according to a preferred embodiment of thepresent invention is shown in FIGS. 1A and 1B. FIG. 1B shows the sensor100 mounted on a circuit board K. The sensor 100 includes onetransmission device S1 and four reception devices R1-R4, which areintegrated on the same semiconductor substrate 10. FIG. 2A shows anultrasonic element 90 for providing each of the transmission device S1and the reception device R1-R4.

The ultrasonic element 90 is similar to the MEMS type ultrasonic element90R as the reception device shown in FIG. 13A. The transmission deviceS1 of the ultrasonic element 90 has the same construction of thereception device R1-R4 of the ultrasonic element 90.

The ultrasonic element 90 is formed of a SOI (i.e.,silicon-on-insulator) semiconductor substrate 10. The substrate 10includes a first semiconductor layer 1 a as a supporting layer, anembedded oxide layer 1 b, a second semiconductor layer 1 c and aprotection oxide film 1 d. A membrane M as a thin portion of thesubstrate 10 is formed by using a semiconductor micromachining method. Apiezoelectric vibrator 20 is formed on the membrane M to cover themembrane M. The piezoelectric vibrator 20 includes a piezoelectric thinfilm 2 and a pair of metallic electrodes 3 a, 3 b. Specifically, thepiezoelectric thin film 2 is sandwiched by a pair of the metallicelectrodes 3 a, 3 b, which are formed of a metallic film.

When the ultrasonic element 90 is used as the transmission device S1,alternating voltage is applied to the metallic electrodes 3 a, 3 b ofthe piezoelectric vibrator 20 so that the membrane M together with thepiezoelectric vibrator 20 is resonated with a predetermined ultrasonicfrequency. Thus, the ultrasonic wave is transmitted. When the ultrasonicelement 90 is used as the reception device R1-R4, the returnedultrasonic wave reflected by the object to be measured resonates themembrane M together with the piezoelectric vibrator 20 so that thereturned ultrasonic wave is converted to an electric signal by thepiezoelectric vibrator 20. Thus, the ultrasonic wave is received.

When the ultrasonic element 90 is used as the transmission device S1, itis preferred that a planar area of the membrane M in the transmissiondevice S1 is comparatively large. This is because it is required togenerate large ultrasonic sound pressure outputted from the transmissiondevice S1. Thus, it is preferred that the planar area of the membrane Min the transmission device S1 is larger than that in the receptiondevice R1-R4. Thus, the transmission device S1 can transmit theultrasonic wave having large sound pressure. However, the planar area ofthe membrane M in the reception device R1-R4 may be comparatively smallas long as the reception device R1-R4 has sufficient sensitivity of theultrasonic wave.

FIG. 3 shows another ultrasonic sensor 100 a according to the preferredembodiment of the present invention. In this case, the planar area ofthe membrane Ms in the transmission device S1 a is larger than theplanar area of the membrane Mr in the reception device R1-R4.

FIGS. 4A to 8C show other ultrasonic elements 91-95 for using as thetransmission device S.

The ultrasonic element 91 shown in FIGS. 4A and 4B includes thesemiconductor substrate having SOI structure. The piezoelectric vibrator21 is formed on the membrane M formed to be a thin portion of thesubstrate 10. The piezoelectric vibrator 21 covers the membrane M. Thepiezoelectric vibrator 21 also includes the piezoelectric thin film 2and the metallic electrodes 3 a, 3 b. The piezoelectric thin film 2 issandwiched by the metallic electrodes 3 a, 3 b.

In the piezoelectric vibrator 21 of the ultrasonic element 91, thepiezoelectric thin film 2 includes a partial cutting pattern 2 a as agroove, which separates the piezoelectric thin film 4 into four parts.This partial cutting pattern 2 a is obtained by removing a part of thepiezoelectric thin film 2, at which a stress caused by radial directionvibration of the membrane M is concentrated. Therefore, rigidity of thepart of the piezoelectric thin film 2 as a stress concentration regionis reduced, so that the membrane M is easily bent, i.e., the flexibilityof the membrane M is increased. Accordingly, the piezoelectric vibrator21 can transmit, i.e., output the ultrasonic wave having sufficientsound pressure.

In the piezoelectric vibrator 22 of the ultrasonic element 92 shown inFIGS. 5A and 5B, the piezoelectric thin film 2 includes a partialconcavity pattern 2 b as a partial groove. The thickness of the part ofthe piezoelectric thin film 2, which is the stress concentration regionof the radial direction vibration of the membrane M, is reduced so thatthe partial concavity pattern 2 b is formed. Thus, the flexibility ofthe membrane M is increased so that the piezoelectric vibrator 21 canoutput the ultrasonic wave having sufficient sound pressure.

The piezoelectric vibrator 23 of the ultrasonic element 93 shown inFIGS. 6A to 6C is formed of multiple layers composed of multiplepiezoelectric thin films 2 and multiple metallic electrodes 3 a-3 c,which are alternately stacked. When the voltage is applied to thepiezoelectric vibrator 23, deformation of the piezoelectric vibrator 23is increased. Thus, vibration amplitude of the membrane M is increasedso that the vibrator 23 outputs the ultrasonic wave having sufficientsound pressure.

In the piezoelectric vibrator 24 of the ultrasonic element 94 shown inFIGS. 7A and 7B, the piezoelectric vibrator 24 and the membrane Md arecantilevered with the substrate 10. Thus, the membrane Md can bedeformed sufficiently, i.e., no portion of the membrane Md, whichprevents the membrane Md from deforming, exists in the membrane Md.Thus, when the voltage is applied to the piezoelectric vibrator 24 sothat the piezoelectric vibrator 24 is deformed, the membrane Md is alsodeformed largely. Accordingly, the vibrator 24 outputs the ultrasonicwave having sufficient sound pressure.

In the piezoelectric vibrator 25 of the ultrasonic element 95 shown inFIGS. 8A to 8C, the membrane Me is formed in such a manner that a partof the embedded oxide layer 1 b of the substrate 10 is hollowed, i.e.,cut from a top surface side of the substrate 10 by a sacrifice etchingmethod. A whole H for the sacrifice etching method is formed around themembrane Me and a beam Ha. Accordingly, the periphery of the membrane Meis partially supported on the substrate 10 through the beam Ha. Thus,the interference part of the membrane Me, which prevents the membrane Mefrom deforming, becomes small. When the voltage is applied to thepiezoelectric vibrator 25 so that the membrane Me is deformed,distortion of the beam Ha is generated and the beam Ha is deformedlargely; and therefore, the membrane Me is largely deformed. Thus, thevibrator 25 outputs the ultrasonic wave having sufficient soundpressure.

Since each ultrasonic element 91-95 can output the ultrasonic wavehaving sufficient sound pressure, the element 91-95 can provide thetransmission device S1 of the ultrasonic sensor 100 having highdetection accuracy. Here, the element 91-95 may also provide thereception device R1-R4 of the ultrasonic sensor 100.

Next, a method for detecting the object by using the ultrasonic sensor100 is explained with reference to FIGS. 9A to 9C. In FIGS. 9A to 9C,the substrate surface of the ultrasonic sensor 100 is disposed to beperpendicular to the ground. Specifically, the surface of thetransmission device S1 is perpendicular to the ground. Here, a X-Y planein FIG. 9A is parallel to the ground. A Z-axis in FIG. 9B isperpendicular to the ground. FIG. 9A shows the reception devices R1, R2of the ultrasonic sensor 100 and the reception ultrasonic wave in theX-Y plane. Specifically, the ultrasonic wave transmitted from thetransmission device S1 is reflected by the obstacle 50, and then thereflected ultrasonic wave is received by the reception device R1, R2 asthe reception ultrasonic wave. FIG. 9B shows the reception devices R1,R3 of the ultrasonic sensor 100 and the reception ultrasonic wave in theZ-X, Y plane. Here, the Z-X, Y plane in FIG. 9B is perpendicular to theground. ΔL represents difference of a path of the reception ultrasonicwave. FIG. 9C is a timing chart showing an alternate pulse signal of theultrasonic wave outputted from the transmission device S1 and fouralternate pulse signals of the ultrasonic wave received by fourreception devices R1-R4.

In FIG. 9A, Dx represents a distance between the center of theultrasonic sensor 100 and the obstacle 50 in the X-Y plane. The distanceDx is calculated on the basis of a S signal No. 1 outputted from thetransmission device S1, a R signal No. 1 received by the receptiondevice R1 and a R signal No. 2 received by the reception device R2. Thereception devices R1, R2 are disposed on an upper side of the sensor 100in FIG. 1. Specifically, the distance Dx is calculated from an averagetime difference ΔTx between reception times (i.e., an arrival time) ofthe R signals No. 1 and No. 2 and a transmission time (i.e., an outputtime) of the S signal No. 1.

In FIG. 9A, θx represents a direction angle to the obstacle 50 in theX-Y plane. The direction angle θx is measured from the X-axis as areference axis. The direction angle θx is obtained on the basis of the Rsignals No. 1 and No. 2 from the reception devices R1 and R2.Specifically, the direction angle θx is calculated from a phasedifference ΔPx between the R signal No. 1 and the R signal No. 2.

In FIG. 9B, Dz represents a distance between the center of theultrasonic sensor 100 and the obstacle 50 in the Z-X, Y plane, which isperpendicular to the ground. The distance Dz is calculated on the basisof the S signal No. 1 from the transmission device S1, the R signal No.1 from the reception device R1 and a R signal No. 3 received by thereception device R3. The reception devices R1, R3 are disposed on a leftside of the sensor 100 in FIG. 1. Specifically, the distance Dz iscalculated from an average time difference ΔTz between reception timesof the R signals No. 1 and No. 3 and the transmission time of the Ssignal No. 1.

In FIG. 9B, θz represents a direction angle to the obstacle 50 in theZ-X, Y plane. The direction angle θz is measured from the X-Y plane as areference plane. The direction angle θz is obtained on the basis of theR signals No. 1 and No. 3 from the reception devices R1 and R3.Specifically, the direction angle θz is calculated from a phasedifference ΔPz between the R signal No. 1 and the R signal No. 3.

On the basis of the distances Dx, Dz and the direction angles θx, θz,the distance between the obstacle 50 and the sensor 100 and thedirection to the obstacle 50 are determined. Thus, the sensor 100detects the obstacle 50.

In the sensor 100, the transmission device S1 and the reception devicesR1-R4 are integrated into the same substrate 10. Accordingly, thedimensions of the sensor 100 and the manufacturing cost of the sensor100 are reduced, compared with the sensor 900 shown in FIG. 13B, inwhich the transmission device S1 and the ultrasonic allay device A90Rare independently formed. Further, since the positioning relationshipbetween the transmission device S1 and the reception device R1-R4 isaccurately designed, i.e., determined on the substrate 10. Thus, evenwhen the sensor 100 is mounted on a bumper of an automotive vehicle,mounting accuracy of the sensor 100 on the bumper does not affect thedetection accuracy of the sensor 100.

Even when the number of the transmission devices S1 and/or the number ofthe reception devices R1-R4 are increased or reduced, and/or even whenthe dimensions of the transmission device S1 and/or the dimensions ofthe reception device R1-R4 are changed, the sensor 100 can be formedonly by changing a mask. Thus, the manufacturing cost of the sensor 100is almost the same.

Although the sensor 100 includes four reception devices R1-R4, theobstacle 50 can be detected by using three reception devices R1-R3.Specifically, the distance Dx in the X-Y plane and the direction angleθx measured from the X-axis are obtained by using two reception devicesR1, R2, which are disposed on the upper side of the sensor 100. Thedistance Dz in the Z-X, Y plane and the direction angle θ z measuredfrom the X-Y plane are obtained by using two reception devices R1, R3,which are disposed on the left side of the sensor 100.

However, the distance Dx in the X-Y plane and the direction angle θxmeasured from the X-axis can be obtained by using two reception devicesR3, R4, which are disposed on a lower side of the sensor 100. Thedistance Dz in the Z-X, Y plane and the direction angle θz measured fromthe X-Y plane can be obtained by using two reception devices R2, R4,which are disposed on the right side of the sensor 100. Thus, theobstacle 50 can be detected by three reception devices R2-R4.

Accordingly, in the sensor 100, two different distances and twodifferent direction angles to the obstacle 50 are obtained. By comparingthese two data of the obstacle 50, operation failure of the sensor 100is judged. Specifically, when two data of the obstacle do not coincide,the operation failure of the sensor 100 occurs. Accordingly, the sensor100 has operation failure detection function.

If the sensor 100 determines that only one reception device R1-R4 actsup the operation failure, the sensor 100 can detect the obstacle 50 byusing other three reception devices R1-R4. Accordingly, the sensor 100has fail safe function.

Further, even when the sensor 100 includes only three reception devicesR1-R3, the sensor 100 can have the operation failure detection function.Specifically, the distance Dx and the direction angle θx are obtainedfrom two reception devices R1, R2, and the distance Dz and the directionangle θz are obtained by using two reception devices R1, R3.Accordingly, the obstacle 50 is detected on the basis of two combinationdata, one of which is obtained from the reception devices R1, R2, andthe other one of which is obtained from the reception devices R1, R3.The other combination data obtained from the reception devices R2, R3can be used for checking the calculation of detection of the obstacle50. Thus, even when the sensor 100 includes three reception devicesR1-R3, the sensor 100 can have the operation failure function.

Thus, when the sensor 100 includes three or more reception devicesR1-R3, the sensor 100 has the operation failure function. When thesensor 100 includes four or more reception devices R1-R4, the sensor 100has the fail safe function. Thus, if the operation failure of the sensor100 is occurred by waterdrop or dust, which is attached to the sensor100, the sensor 100 can avoid the operation failure.

The sensor 100 can output two or more different ultrasonic waves havingdifferent frequencies, which are transmitted from one transmissiondevice S1 by controlling the frequency of the alternate pulse signal interms of time, the pulse signal being applied to the transmission deviceS1. By using two different ultrasonic waves, the sensor 100 can detectthe obstacle 50 with humidity compensation function. Here, the inputvoltage is controlled to have a frequency range other than the resonantfrequency of the membrane M so that the ultrasonic waves having twodifferent frequencies are transmitted.

FIG. 10 explains the method for compensating the humidity. In FIG. 10,the transmission device S1 outputs two different ultrasonic waves havingtwo different frequencies f1, f2. The transmission device S1 transmitsthe first ultrasonic wave having the first frequency f1, and then, thedevice S1 transmits the second ultrasonic wave having the secondfrequency f2. The first and the second ultrasonic waves areperiodically, i.e., with a predetermined time interval, outputted. Infour reception devices R1-R4, the first R signal No. 1 corresponding tothe first ultrasonic wave and the second R signal No. 1 corresponding tothe second ultrasonic wave to the first R signal No. 4 corresponding tothe first ultrasonic wave and the second R signal No. 4 corresponding tothe second ultrasonic wave are detected. The relationship among thefirst R signals No. 1-4 and the first S signal No. 1 corresponding tothe first ultrasonic wave in FIG. 10 is the same as that in FIG. 9C.Further, the relationship among the second R signals No. 1-4 and thesecond S signal No. 1 corresponding to the second ultrasonic wave inFIG. 10 is the same as that in FIG. 9C.

In FIG. 10, the height of the alternate pulse signal of the first Ssignal No. 1 of the first frequency f1 is equal to that of the second Ssignal No. 1 of the second frequency f2. However, the height of thefirst R signal No. 1 of the first frequency f1 is higher than that ofthe second frequency f2, i.e., the second R signal No. 1 of the secondfrequency f2 is largely attenuated, compared with the first R signal No.1 of the first frequency f1. Similarly, the second R signals No. 2-4 arelargely attenuated, i.e., reduced.

Here, attenuation loss P, i.e., absorption loss of the ultrasonic waveis obtained by the following formula.

$\begin{matrix}{P \propto {\mathbb{e}}^{- {mr}}} & \left( {F\; 1} \right) \\{m = {{\left( {33 + {0.2T}} \right){f^{2} \times 10^{- 12}}} + \frac{Mf}{{{k/2}\pi\; f} + {2\pi\;{f/k}}}}} & ({F2}) \\{k = {1.92 \times \left( {\frac{G_{0}}{G} \times h} \right)^{1.3} \times 10^{5}}} & ({F3})\end{matrix}$

Here, m represents absorption coefficient, r represents transmissiondistance, M represents a predetermined coefficient, f represents afrequency, T represents a temperature, G₀ represents a saturated vaporpressure, G represents a total air pressure, and h represents ahumidity.

From the above formula F1, the attenuation loss P depends on thefrequency f. As the frequency f of the ultrasonic wave becomes larger,the attenuation loss becomes larger. Further, the attenuation loss Pdepends on not only the frequency but also the temperature T and thehumidity h of the transmission environment. The frequency f of theultrasonic wave is preliminarily determined. The temperature T of theenvironment can be detected by an external temperature sensor or thelike. When the sensor 100 is mounted on the vehicle, the temperature T,i.e., the external temperature can be detected easily. However, thehumidity h of the environment, i.e., the external humidity h is notdetected easily by a humidity sensor. This is because there is noappropriate humidity sensor for detecting the external humidity aroundthe vehicle.

However, since the received ultrasonic waves having two differentfrequencies f1, f2 are measured, the humidity h can be calculated on thebasis of the difference of two attenuation losses P obtained from twodifferent frequencies f1, f2. This calculated humidity h is used forcompensating the standard humidity, which is preliminarily determinedand memorized in the sensor 100. Thus, the sensor 100 has the humiditycompensation function. In this case, the detection accuracy of thesensor 100 is much improved regarding the humidity change.

Although the sensor 100 includes only one transmission device S1, it ispreferred that the sensor 100 includes two or more transmission devicesS1. When the sensor 100 includes two transmission devices S1, eachtransmission device S1 can output the ultrasonic wave having differentfrequency with high Q value, the device S1 outputting the wave by usingdifferent resonant frequency of the membrane M.

FIG. 11 shows an ultrasonic sensor 101 having two transmission devicesS1, S2. The sensor 101 can output two ultrasonic waves having differentfrequencies f1, f2 simultaneously by using two transmission devices S1,S2 for outputting two different ultrasonic waves. Thus, no compensationfor compensating motion of the vehicle is necessitated. Here, since theultrasonic waves having different frequencies f1, f2 have the sametransmission velocity, the reflected ultrasonic waves are arrived at thesensor 100 at the same time. Accordingly, frequency analysis fordecomposing the reception ultrasonic waves into the component having thefirst frequency f1 and the component having the second frequency f2 isrequired.

FIG. 12 shows an ultrasonic sensor 102 having the transmission device S1and eight reception devices R1-R8. The transmission device S1 issurrounded with eight reception devices R1-R8. In this case, it ispreferred that two reception devices R1-R8 are arranged to besymmetrically with respect to the transmission device S1. Specifically,a pair of the reception devices R1, R8, a pair of the reception devicesR2, R7, a pair of the reception devices R3, R6, and a pair of thereception devices R4, R5 are arranged to be symmetrically with respectto the transmission device S1 so that each pair of the reception devicesR1-R8 surrounds the transmission device S1.

In this case, since each pair of the reception devices R1-R8 issymmetrically disposed, the reflected ultrasonic wave outputted from thetransmission device S1 is returned to the pair of the reception devicesR1-R8 in such a manner that the sound pressure of the receivedultrasonic wave received by one of the pair of the reception devicesR1-R8 is almost the same as the other one of the pair of the receptiondevices R1-R8. Accordingly, the detection accuracy of the obstacle 50 isimproved.

Thus, each sensor 100, 100 a, 101, 102 has small dimensions and lowmanufacturing cost, and the detection accuracy of the sensor 100, 100 a,101, 102 is not affected by mounting accuracy of the sensor on thevehicle. Further, the sensor 100, 100 a, 101, 102 has high detectionaccuracy, even if the waterdrop or the dust is adhered to the sensor100, 100 a, 101, 102 and even if the humidity around the sensor 100, 100a, 101, 102 changes.

Although the sensor 100, 100 a, 101, 102 includes one transmissiondevice S1 and four or eight reception devices R1-R8, the sensor mayincludes one or more transmission devices S1 and two or more receptiondevices. When the sensor includes multiple transmission devices andmultiple reception devices, the information from the sensor isincreased. Further, when the sensor includes two or more transmissiondevices, the sound pressure of the ultrasonic wave becomes larger, andthe directivity of the ultrasonic wave is controlled.

Alternatively, the reception devices in the sensor may be arrayed sothat a transmission signal is received by multiple reception devices inorder to cancel the transmission signal, since the transmission signalmay cause noise of the sensor. Specifically, when the transmissiondevice and the reception device are integrated into one substrate, thetransmission signal may input into the reception device so that thetransmission signal may cause the noise of the sensor. Thus, bycanceling the inputted transmission signal, the noise of the sensor isreduced. Accordingly, when the obstacle is disposed near the sensor, theS/N ratio of the signal is improved for detecting the obstacle.

Although the reception device includes the piezoelectric thin film sothat the reception device provides a piezoelectric type device, thereception device may be a capacitance type device for detecting acapacitance change between electrodes. Further, the reception device maybe a piezo type for detecting an output of a gauge generated bypressure. Furthermore, the sensor may include a combination of thesedifferent type reception devices.

While the invention has been described with reference to preferredembodiments thereof, it is to be understood that the invention is notlimited to the preferred embodiments and constructions. The invention isintended to cover various modification and equivalent arrangements. Inaddition, while the various combinations and configurations, which arepreferred, other combinations and configurations, including more, lessor only a single element, are also within the spirit and scope of theinvention.

1. An ultrasonic sensor for detecting an object, comprising: asubstrate; a transmission device for transmitting an ultrasonic wave; aplurality of reception devices for receiving the ultrasonic wave; and acircuit for processing received ultrasonic waves, which are received bythe reception devices after the ultrasonic wave transmitted from thetransmission device is reflected by the object, wherein the transmissiondevice and the reception devices are integrated into the substrate, thenumber of the reception devices is equal to or larger than three so thatthe circuit is capable of detecting an operation failure, each of thetransmission device and the three reception devices has a surface fortransmitting or receiving the ultrasonic wave, the surface beingperpendicular to a ground, the three reception devices are composed of afirst to a third reception devices, the first reception device isdisposed above the third reception device, and disposed on a left sideof the second reception device, the circuit is capable of calculating ahorizontal plane distance between the object and the sensor in ahorizontal plane parallel to the ground and a horizontal plane directionangle from the sensor to the object in the horizontal plane on the basisof the received ultrasonic waves received by the first and the secondreception devices, the circuit is further capable of calculating avertical plane distance between the object and the sensor in a verticalplane perpendicular to the ground and a vertical plane direction anglefrom the sensor to the object in the vertical plane on the basis of thereceived ultrasonic waves received by the first and the third receptiondevices, and the circuit is capable of checking the horizontal and thevertical plane distances and the horizontal and the vertical planedirection angles on the basis of the received ultrasonic waves receivedby the second and the third reception devices so that the circuit iscapable of detecting the operation failure.
 2. An ultrasonic sensor fordetecting an object, comprising: a substrate; a transmission device fortransmitting an ultrasonic wave; a plurality of reception devices forreceiving the ultrasonic wave; and a circuit for processing receivedultrasonic waves, which are received by the reception devices after theultrasonic wave transmitted from the transmission device is reflected bythe object, wherein the transmission device and the reception devicesare integrated into the substrate, the number of the reception devicesis equal to or larger than four, each of the transmission device and thefour reception devices has a surface for transmitting or receiving theultrasonic wave, the surface being perpendicular to a ground, the fourreception devices are composed of a first to a fourth reception devices,the first reception device is disposed above the third reception device,and disposed on a left side of the second reception device, the fourthreception device is disposed under the second reception device, anddisposed on a right side of the third reception device, the circuit iscapable of calculating a horizontal plane distance between the objectand the sensor in a horizontal plane parallel to the ground and ahorizontal plane direction angle from the sensor to the object in thehorizontal plane on the basis of the received ultrasonic waves receivedby the first and the second reception devices, and further capable ofcalculating a vertical plane distance between the object and the sensorin a vertical plane perpendicular to the ground and a vertical planedirection angle from the sensor to the object in the vertical plane onthe basis of the received ultrasonic waves received by the first and thethird reception devices, so that a first data of the object is obtained,the circuit is capable of calculating the horizontal plane distance andthe horizontal plane direction angle on the basis of the receivedultrasonic waves received by the third and the fourth reception devices,and further capable of calculating the vertical plane distance and thevertical plane direction angle on the basis of the received ultrasonicwaves receive by the second and the fourth reception devices, so that asecond data of the object is obtained, and the circuit is capable ofchecking the first data and the second data so that the circuit iscapable of detecting the operation failure.
 3. An ultrasonic sensor fordetecting an object, comprising: a substrate; a transmission device fortransmitting an ultrasonic wave; a plurality of reception devices forreceiving the ultrasonic wave; and a circuit for processing receivedultrasonic waves, which are received by the reception devices after theultrasonic wave transmitted from the transmission device is reflected bythe object, wherein the transmission device and the reception devicesare integrated into the substrate, and wherein each of the transmissiondevice and the reception devices is provided by an ultrasonic element,the ultrasonic element is disposed on a membrane of the substrate, theultrasonic element includes a piezoelectric thin film and a pair ofmetallic electrodes so that a piezoelectric vibrator is provided, thepiezoelectric thin film is sandwiched by the metallic electrodes, thepiezoelectric vibrator is capable of resonating together with themembrane at a predetermined ultrasonic frequency, the transmissiondevice has a planar area of the membrane, and each reception device hasa planar area of the membrane, which is smaller than that of thetransmission device.